The present invention relates to a tire with radial carcass reinforcement which is intended to bear heavy loads, and more particularly to a xe2x80x9cheavy-vehiclexe2x80x9d-type tire, intended to be fitted on vehicles such as, for example, lorries, road tractors, buses, trailers and others, and more particularly to the novel reinforcement structure for the beads of said tire.
Generally, a tire of the type in question comprises a carcass reinforcement formed of at least one ply of metal cables, which is anchored in each bead to a bead wire, forming an upturn. The carcass reinforcement is radially surmounted by a crown reinforcement, composed of at least two plies of metal cables which are crossed from one ply to the next, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction. The carcass reinforcement upturns are generally reinforced by at least one bead reinforcement ply formed of metal cables which are oriented at a small angle relative to the circumferential direction, generally of between 10xc2x0 and 30xc2x0.
The bead reinforcement ply is generally located axially to the outside, along the carcass reinforcement upturn, with a radially upper end located above or below the radially upper end of the carcass reinforcement upturn. As for the radially lower end of such a reinforcement ply, it is generally located beneath a straight line parallel to the axis of rotation and passing approximately through the center of gravity of the meridian section of the anchoring bead wire of the carcass reinforcement.
The known solution aims to avoid deradialization of the cables of the carcass reinforcement upturn and to minimize the radial and circumferential deformations to which the end of said upturn and the outer rubber layer covering the bead and providing the connection to the rim are subjected.
The life of xe2x80x9cheavy-vehiclexe2x80x9d tires, owing to the progress achieved, and to the fact that certain types of travel are made less of a handicap as far as wear of the tread is concerned, has become such that it is also necessary to improve the endurance of the beads. Said improvement must focus on the degradation of the rubber layers at the level of the ends of the carcass reinforcement upturn and the radially outer ends of the bead reinforcement plies. More particularly in the case of tires which are subject to prolonged travel, which travel frequently induces a high temperature of the beads, owing to the temperatures which the mounting rims reach, the rubber mixes in contact with the rim are then subject to a reduction in their rigidity, and to more or less slow oxidation, hence the very marked tendency of the carcass reinforcement to unwind from around the bead wire under the action of the internal inflation pressure, despite the presence of one or more bead reinforcement plies. There then arise bead wire movements and shearing deformations at all the ends of the plies, resulting in the destruction of the bead. Said improvement must also, and primarily, focus on this second possibility of degradation.
U.S. Pat. No. 3,301,303, in order to improve the endurance of the bead zone of a tire bearing heavy loads, claims a carcass reinforcement which is wound in a precise trace around two bead wires which are practically axially adjacent: the carcass reinforcement is first anchored by turning up about the axially innermost bead wire, passing radially from the outside to the inside, then axially from the inside to the outside, then passing radially below the second bead wire, which is arranged axially to the outside, to wind about said second bead wire, passing radially from the inside to the outside then axially from the outside to the inside to form an upturn which comes back radially beneath the first bead and possibly winds around said bead wire to then be arranged along the axially outer face of the carcass ply.
The carcass reinforcement of a radial tire, mounted on its operating rim and inflated to the recommended pressure, has in one sidewall a regularly convex meridian profile between approximately the zones of connection firstly with the meridian profile of the crown reinforcement and secondly with the bead. In particular, starting from the radius where the carcass reinforcement is subject to the influence of the bead reinforcement plies, said reinforcement has in the bead a meridian profile which is either substantially rectilinear or curved in the opposite direction to the curvature in the sidewalls, that is to say, substantially parallel to the curvature of the rim flanges starting from a point of inflection located radially approximately at the level of the radially upper end of the bead reinforcement ply placed along the carcass reinforcement upturn.
Such an arrangement associating two bead wires within a bead with a meridian profile having a point of inflection in the region of the bead significantly improves the endurance of the beads in the case of travel on a heated rim, but becomes insufficient, however, in the case of travel where the supported loads become greater, or the inflation pressures less, than the recommended loads and pressures, and more particularly when the ratio of the height H on rim to the maximum axial width S of the tire becomes less than 0.8.
Research has led to the conclusion that the meridian profile of the carcass reinforcement in the region of the change of curvature within the bead needed to be reinforced by at least one bead reinforcement armature.
In order to improve the endurance of the beads of a tire having an HIS form ratio of less than 0.8, intended to be fitted on a vehicle bearing heavy loads, said tire, according to the invention, comprises at least one radial carcass reinforcement, formed of at least one ply of inextensible reinforcement elements, anchored within each bead B to at least two bead wires which are close to each other, which is turned up about the first and then wound around the second to form an upturn, and it is characterized in that, viewed in meridian section, an additional reinforcement armature, formed of at least one ply of circumferential reinforcement elements, is placed along the carcass reinforcement, at least axially to the inside, in the bead region where the trace of the meridian profile of said carcass reinforcement changes curvature to become rectilinear or concave at the point of tangency T with the virtually circular coating layer of the first anchoring bead wire, said reinforcement armature having its radially lower end radially beneath the straight line Dxe2x80x2 which is parallel to the axis of rotation and passes through that point of the coating layer of the first anchoring bead wire which is radially farthest from the axis of rotation, but above the straight line D which is parallel to the axis of rotation and passes through that point of the coating layer of the first anchoring bead wire which is radially closest to the axis of rotation.
The additional reinforcement armature will preferably have its radially upper end located at a radial distance from the straight line D which lies between a quantity equal to half the radial distance between the straight lines D and Dxe2x80x3 increased by half the radial distance between the straight lines D and Dxe2x80x2 and a quantity equal to half the radial distance between the straight lines D and Dxe2x80x3 reduced by half the radial distance between the straight lines D and Dxe2x80x2 , the straight line Dxe2x80x3 being the straight line at the point of greatest axial width.
The straight line Dxe2x80x3 at the point of greatest axial width is, by definition, the straight line parallel to the axis of rotation and passing through that point in the meridian profile of the carcass reinforcement which corresponds to the point of greatest axial width when the tire is mounted on its operating rim and inflated to the recommended, non-loaded pressure.
The elements of the additional reinforcement armature are said to be circumferential if the angle which they form with the circumferential direction is between +5xc2x0 and xe2x88x925xc2x0.
Whatever the path which the carcass reinforcement follows in its anchoring to the two bead wires, for example the trace as described in U.S. Pat. No. 3,301,303 and defined above, or alternatively a trace defined by a first upturn about the bead wire axially farthest to the inside, going from the inside to the outside, then passing axially between the two bead wires to be radially above the axially outermost bead wire, then to turn up about said bead wire, going radially from the top to the bottom and axially from the outside to the inside, and then being located radially below the innermost bead wire, it is advantageous for the end of the upturn to be located radially beneath the axially innermost bead wire.
One advantageous modification of the bead thus obtained resides in the fact that the straight line joining the two centers of gravity of the cross-sections of bead wires is no longer parallel to the axis of rotation, but forms with the latter an angle xcex2, which is open axially and radially to the outside, of between 20xc2x0 and 60xc2x0.
This structure thus permits partial takeup of the meridian tensile forces to which the carcass reinforcement is subjected, and thus minimizes the possible deformations of the carcass reinforcement upturn, whatever the conditions of travel.
The takeup of the forces will be all the greater the greater the number of reinforcement elements of the additional armature; they will preferably be metal, made of steel, which are adjoining over the entire length of the reinforcement ply. Reinforcement elements are said to be adjoining if, in the direction perpendicular to said elements, the distance between two adjacent elements is reduced as much as possible.
The takeup of the meridian tensile forces of a carcass reinforcement is known per se from numerous documents. French Patent 750 726 teaches gluing carcass reinforcement plies to auxiliary reinforcements, each formed of a cord wound in a spiral and consequently having significant flexibility in the transverse direction. Said gluing is effected such that two carcass reinforcement plies tightly surround an auxiliary reinforcement, this surrounding being in the absence of a bead wire and a carcass reinforcement upturn.
French Patent 1 169 474 also teaches dispensing with the bead wire usually used by small plies of metal cords or cables inclined on the parallel hooking line, the angle of inclination possibly being as low as 5xc2x0, the carcass reinforcement ply (plies) being turned up or not turned up.
French Patent 1 234 995 provides for the replacement of the usually oblique reinforcement elements of the bead reinforcement ply (plies) which is (are) intended to reduce the movements of hooking against the rim flange with circumferential elements, the reinforcement armature possibly being arranged between the main part of the carcass reinforcement and the upturn of said reinforcement, or axially to the outside of said carcass reinforcement upturn.
The same bead reinforcement armatures with circumferential elements are found in French Patent 1 256 432, said elements being intended to take up the tensile forces of the carcass reinforcement in their entirety, and in the case of tires for passenger vehicles, in the absence of anchoring bead wires or any other reinforcement ply.
FR 2 055 988 describes a certain number of possible applications of a bead reinforcement armature having circumferential elements, in particular the use of such an armature in the place of the conventional armature having oblique elements which are arranged axially to the outside of the carcass reinforcement upturn.
The problems of endurance referred to above, which are influenced by the operating temperature of the beads, have been solved to a very slight degree by what is called a xe2x80x9cthinnedxe2x80x9d bead structure, obtained, for example, by the means described in French Patent 2 451 016 which, to this end, teaches to impart a relatively great concavity to the outer sidewall of the tire, between the point where said sidewall is no longer in contact with the rim and the point where said sidewall achieves its maximum distance relative to the equatorial plane, when the tire is mounted on its operating rim and inflated to the recommended pressure.
The carcass reinforcement, in order to reduce the volume of the beads as far as possible, is advantageously formed of reinforcement elements of textile material, which makes it possible to produce the reinforcement windings around the bead wires easily. However, sometimes, for various reasons, the presence of a metal carcass reinforcement is required, in particular in the regions of the crown and sidewalls of the tire. One preferred solution of the invention then consists in providing said tire with a carcass reinforcement made of three parts: a first part consisting of metal reinforcement elements passing at least beneath the crown reinforcement and into the sidewalls, and two other parts, formed of textile reinforcement elements, which are turned up and wound around the two anchoring bead wires of each bead. The edges of the metal part, which part may or may not be turned up about the anchoring bead wires, have with the axially inner edges of the textile parts a common overlap length, at least located in the region in which the additional armature of circumferential reinforcement elements is located.
The endurance of the beads according to the invention may also be improved by thinning said beads, imparting to the carcass reinforcement anchored to two bead wires within a bead a meridian profile which does not have any change in curvature in the region of the beads. Said meridian profile is then characterized in that, viewed in meridian section, its trace, between the point A of greatest axial width and the point of tangency T with the virtually circular coating layer of the first anchoring bead wire, is convex and circular over its entire length, the center of curvature being located on the straight line Dxe2x80x3 , of greatest axial width, such that the thickness s of the bead B, measured on a line perpendicular to said meridian profile at a point C of height hc of between 30 and 40 mm and measured relative to the base of the bead YYxe2x80x2, is between 2 and 2.5 times the thickness e of the sidewall measured on the straight line Dxe2x80x3 of greatest axial width.
Not only does the combination of the presence of two anchoring bead wires per bead with the presence of a bead reinforcement armature, having circumferential elements, and arranged, according to the invention, within the main part of the carcass reinforcement, permit improvement of the endurance of the beads, but furthermore, in combination with the entirely convex meridian profile of the carcass reinforcement described above, it makes it possible to obtain a very significant reduction in the weight of the tire, while not adversely affecting its properties. Said reduction in weight may be advantageously accentuated by the replacement of the bead wires having rectangular metal wires, which are generally used in the type of tire in question, by more economical bead wires of xe2x80x9cbraidedxe2x80x9d type, that is to say, formed of a helicoidal braiding of a plurality of metal wires, or bead wires of the xe2x80x9cstackxe2x80x9d type of polygonal section, that is to say bead wires obtained by winding a metal wire on a form, which results in a plurality of layers of different widths. The types of bead wire above furthermore mean that less space is taken up, owing to the shape of their cross-sections and to the possible reduction of said sections, of the order of 30% and 50% for each, respectively, which, relative to the application of the invention to the case of bead wires with rectangular wires, which take up more space, results in a thinner bead thickness.
The bead reinforcement armature thus described may be produced separately by spiral winding on a horizontal support of suitable shape, and then be transferred to the non-vulcanized tire blank. The manufacture of such a tire will however be advantageously facilitated by the use as reinforcement elements of lengths or assemblies or groups of lengths of cables of a circumferential length of between 0.2 and 0.4 times the circumferential length of the reinforcement ply, which permits laying of the bead reinforcement armature on the building drum for the raw blank of the carcass reinforcement and the shaping of said blank into a torus without major difficulties, and said average length being measured upon laying on said building drum. The circumferential gaps or cuts between cut elements will preferably be offset from each other.